NOTE 2 In the CIE 1931 standard colorimetric system, colorimetric purity, pc, is related to excitation purity, pe, by the equation pc=pe yd/y where yd, and y are the y chromaticity coordinates respectively, of the monochromatic stimulus and the colour stimulus considered.

NOTE 3 In the CIE 1964 standard colorimetric system, a measure, pc,10, is defined by the relation given in NOTE 2, but using pe,10, yd,10 and y10 instead of pe, yd, and y respectively.